Could you explain why some of my plastic on clamshell case got wrinkled?

How can I avoid this.

BAsed on my experience, it DID NOT happen to all my Ken's but it happened to some films from German companies such as UFA.

I don't know if this because of the plastic type they used. But I have multi reels from UFA. One case is getting wrinkled but the others are not. So my assumption is that their plastic type is the same.

So I wonder why when they were stred at the same place, same humidity, same temperatur, one got wrinkled.